By Loudon R. Stanford and Vance T. MacKubbin

INTRODUCTION AND HISTORY

The Idaho Geological Survey has been producing geologic maps digitally since 1989. The Idaho Survey began to capture a database of basic geologic attributes associated with each geologic feature on a map in 1992 as part of its map digitizing process. Beginning in 1996, metadata, or information about the map and its sources, has been input for each 30 x 60 minute geologic map compilation. The attributed spatial data can be used in a Geographic Information System (GIS) to perform simple queries or analyses. However, to receive consistent and meaningful answers to complicated questions about the geology on a digital map requires yet another database of map information, derived from the map legend.

Taken together, the spatial map data, map legend, and metadata comprise a digital geologic map database. The design of these data sets and how they relate to one another to supply reasonable interpretations of the map is a digital geologic map data model (Tsichritzis and Lochovshy, 1982).
Currently, many groups are working on geologic map data model design. One of the best known of these to date is the U.S. Geological Survey (USGS), Geological Survey of Canada (GSC), and Association of American State Geologists (AASG) effort: Digital Geologic Map Data Model Version 4.3a (Johnson and others, 1999). For a review of the various geologic map data model efforts see Richard (1999). Currently the North American digital geologic map data model Steering Committee (NADMSC) is forming technical working groups to tackle many of the thorny issues involved in developing a North American Data Model (NADM) for geologic mapping.

The tables that hold map information and which make up the geologic map database are the heart of the model. These tables can be manipulated, related, or translated. One of the goals set by the NADMSC is to develop translating tools which will enable different "flavors" of the data model to be migrated to one central format for exchange and archiving purposes. There is at present no data model standard or implementation protocol.

IMPLEMENTING A GEOLOGIC MAP DATA MODEL

Data Users

Demand for better geologic map data has traditionally come from within the geologic profession. But with the advent of GIS technology, new customers are requesting digital geology data sets to aid in analysis of problems as varied as the relationship of geology to tree nutrition or fresh water fisheries. To meet this need for better and smarter digital geology, the Idaho Survey began work on a data model design and implementation that would be "user friendly" and that could be understood by a non-geologist as well as a geologist.

Which Maps to Apply the Data Model To?

The Idaho Survey is a small agency with limited resources. The benefits of developing and implementing a geologic map data model must be weighed against the costs. The Idaho Survey already has a well established set of procedures for capturing and publishing geologic maps. Implementation of a data model at the Idaho Survey by necessity needed to be developed as an extension to existing procedures and software protocols.

The Idaho Survey has historically produced geologic map compilations in paper format, providing the map customer with a synopsis of the current, best geology for a given area. They also provide an economical format: covering the most geology for the least money. For several years the Survey has been committed to digitally compiling 30 x 60 minute maps. These compilations are constructed using a combination of existing geology and new mapping and are the only maps to which we are presently applying the data model. Geologic map sources for the 30 x 60 minute quadrangles are digitized from source materials at original scale where possible. Each map object receives a source identifier code. In this way, simple queries can determine map sources, and source map metadata, within the compiled geologic map. Several advantages are gained by using 30 x 60 minute compiled geologic map tiles, as the backbone of digital geologic mapping at the Idaho Survey:

Map format fulfills most of our customer's data needs

Map data can be designed for merging with adjoining maps

Maps can be edge matched

Most geology for the money

Idaho Survey procedures already in place can be used (limited re-tooling)

Data Model Design and Additions

The Idaho Survey relied heavily on the USGS/AASG proposed model 4.3a (Johnson and others, 1999) for the design of its data model. Because of our plan to limit the data model to 30 x 60 minute compilations, and because of additional needs, we added some data elements to this structure:

More information, including a complete table flow chart, can be found at the Idaho Survey web site http://www.idahogeology.org/. Look for links to the Digital Geologic Mapping lab (DMG) first, and then the Data Model.

Tool Development Environment and Software Platform

Without software query tools to access and retrieve the information held in the geologic map database, only users with considerable expertise in database manipulation and GIS could work with our data. Even for experts, map data queries would be slow and cumbersome. Good query tools open up the power of the data model to a much larger group of data users.

For many reasons, the Idaho Survey chose to develop the query tools around ArcView GIS. ArcView provides a widely used, relatively easy working environment with a good programing language (Avenue) which allows easy development and data set distribution.

Major Features of the Idaho Survey
Data Model

There are currently three major functions which the Idaho Survey data model tools handle:

Merging tools (spatial join) to merge two or more map compilations (tiles)

Remaining Work

Most of the crucial design decisions have been made for the Idaho Survey geologic map data model and preliminarily implemented. What remains to be done is the polishing of the basic software tools and finalizing and incorporating the non-implemented tables into the Idaho Survey model. Testing of the model in ArcView will begin with in-house geologists and will certainly result in the fine-tuning of elements of the data model and its tools. With the design nearly stable, new tools need to be created that will enable a geologist to enter map legend information.